1. Field of the Invention
The present invention relates generally to a method and apparatus for inspecting the external appearance of objects and, more specifically, to the method and apparatus therefor using a photoelectrical conversion sensor such as a video camera or the like and electronic processors.
2. Description of the Prior Art
In addition to the functional inspection of objects, the inspection of external appearance is very important in the production of the object. External appearance inspections are made for a variety of reasons such as surface inspection, checking for flaws in the surface of the object or for colour irregularities thereof, or for external shape that detect the defects of an object in its external shape.
Normally, in the apparatus for inspecting the external appearance of an object light from a light source illuminates the inspected object, and the reflected light from the object is picked up by a photoelectric conversion sensor such as a video camera in which the optical image of the object is converted into an electrical signal, and this electrical signal is processed by an electronic processor that possesses a specific logic circuitry to thereby judge the good or bad of the object. There are variations of such methods. One of them is a so-called image diffraction method in which, for instance, when the roundness of a circle is to be measured, the center point of the circle is detected, and the distances from such circle center point to respective points on the external periphery of the circle. In other words, the radiuses of the circle at respective points are measured in order to confirm the roundness of the circle. When the external appearance of other shaped objects is inspected, for example, the center of gravity of the object is detected, and the distances from the center of gravity to respective points on the external portion of the object are measured, and the measured distances are compared with a standard value. Otherwise, there is a so-called pattern matching method in which the pattern of an inspected object is compared with the basic pattern (or standard pattern) of the object to thereby judge the good or bad of the inspected object from such differences.
The above described typical methods of the conventional art have the following problems. At first, there is the problem relating to the generality or flexibility of applications thereof. In other words, when the shape of the inspected object is changed or when the items of inspection, or rather the portion or surface to be measured is changed, it is necessary to change the computer software program in the above-mentioned image diffraction method. Otherwise, in anticipation of such change, although such change is backed up on the computer software and an instruction for such change may be given from a keyboard, the processing that accompanies such change becomes complex, and a special type of operator training becomes necessary.
Further, even in the second example, i.e., pattern matching method, processing to change the basic or standard pattern is required as in the case of the image diffraction method. Also, there is the problem of the processing time. Normally, since most requirements for inspection apparatuses are for on line use at real production lines, it is necessary to inspect objects that flow at a speed of more than several hundred pieces per minute, on a conveyor. Thus, there is a limited processing time made available to inspect one inspected object.
Now then, in order to solve the first problem as mentioned above, in other words, in order to increase the flexibility of adaptation, a highly precise software is required, causing the program length thereof to increase, resulting in an increase in the processing time. This counteracts against solving the second problem.